The invention relates generally to radio spectrum use by non-primary users. More specifically, the invention relates to allocating unused portions of the radio spectrum to secondary or additional users.
Wireless communication on the radio spectrum is limited by the amount of unassigned spectrum available to users. Generally, the entire range of the useful spectrum is divided (in the frequency dimension) into blocks or bands of frequencies called allocations. These frequency allocations determine the type of use allowed within the block or band of frequencies. For example, separate allocations are made for broadcasting, land mobile radio, point-to-point microwave, and amateur radio services.
In some, but not all, cases, these allocations are further subdivided into allotments. Broadly, the term refers to the subdivision of bands already allocated to a particular service for specific user and/or provider groups within that service. Within an allocation for the land mobile radio service, for example, allotments might be made for public cellular mobile telephone, mobile radio, and public safety services.
Radio spectrum is commonly assigned to bandwidth users in a zoned fashion, i.e., various primary users are granted reserved blocks of spectrum whether they are continuously transmitting or not. Because few primary users transmit continuously, much of the assigned spectrum is largely quiet. In addition, because the spectrum has historically been used in an analog fashion, the bandwidths of the allocations and/or allotments were distributed in wider bands than may be employed for modern digital communication. The wider bandwidths were assigned in such a manner to prevent interference between adjacent bands in analog communication, which typically involves a higher signal to noise ratio than digital communication and therefore may be more sensitive to interference. Accordingly, the historic allotment of bands was inefficient to some degree. It has therefore been a goal of many telecommunication users to find some way that unused spectrum might be used by secondary users during the periods of non-use by the primary assignees.
Certain proposed approaches for making use of quiet spectrum have been frustrated by complex technical problems including what is termed as “the hidden node” problem in which a non-primary user cannot determine that a recipient of a primary user is receiving. During the transmission, if the non-primary user were to begin transmitting, the primary user's transmission would experience interference. Because of the persistent and unsolved technical problems, national licensing and regulatory authorities have been reluctant to encourage quiet spectrum usage by non-primary users.